This invention relates to a new method of testing of RF transistors in nonlinear operation using automatic RF and low frequency (IF) impedance tuners. The tuners create user defined test impedances and optimize the transistor (DUT) performance at the signal RF (Carrier) and at low modulation (IF) frequency simultaneously and totally independently.
Modern design of RF high power microwave amplifiers, mixers and other components used in various communication systems requires accurate knowledge of the characteristics of the transistors used in the circuits. It is insufficient and inaccurate for the transistors operating at high power in their highly non-linear regions, to be described using analytical or numerical models. Instead the transistors (DUT) need to be characterized using specialized test setups, which use tuners and other test equipment, under the actual operating conditions.
A traditional method for testing and characterizing transistors for high power operation is “load pull” and “source pull” (FIG. 1). Load pull or source pull are measurement techniques employing signal sources, RF impedance tuners (input and output), discrete DUT's in test fixtures or chips on-wafer and other test equipment, like a Power Meter. A control computer recalls previously generated tuner calibration data, positions the tuner's tuning probes to create the desired impedances and reads instrument data via GPIB or other digital communication.
Electro-mechanical microwave slide-screw tuners (see ref. 1) are used in most cases for high power load pull testing, because they have several advantages, such as long-term stability, higher handling of RF power, easier operation and lower cost, compared to other type of tuners such as electronic and active tuners. A benefit of slide screw tuners favoring in particular this kind of combined RF-IF tuning operation, is their low pass behavior, meaning that at low (IF) frequencies these tuners behave like simple 50Ω through transmission lines.
In most cases of wireless communication the RF signal is not pure carrier wave (CW) but includes some kind of modulation, which contains the information to be transmitted. In these cases the signal spectrum spreads around the main carrier wave (RF) and the bandwidth depends on the information contained and the modulation techniques used. Modem telecommunication schemes require sidebands of several MHz up to a few dozens of MHz (FIG. 2). The transistors being driven into the nonlinear regime, the RF upper and lower sideband frequencies (USB+LSB) are mixed down into the IF baseband and from there they interact with the (low frequency) DC biasing circuitry ZIF (FIG. 3) and from there they get mixed up again into the RF signal where they interact with the original modulation signals (FIG. 4). This phenomenon is instantaneous and must be investigated by simultaneous RF and IF impedance control. In the context of available technology RF here means frequencies in the GHz range (0.8 to 18 GHz typically) and IF means the low MHz range (1 to 20 MHz, typically). But the method is valid globally for any two distinct frequency bands.
This invention describes a method, which allows controlling and optimizing not only, as hereto, the RF impedances seen by the DUT in the GHz frequency range, but also, at the same time, the low frequency impedances, at baseband IF frequency in the MHz range; the IF impedances are presented to the DUT essentially through the DC biasing networks which act as frequency diplexers (separators between RF and IF frequencies). Typical LC bias networks, also called Bias Tees, are shown in FIG. 5 (53) and FIG. 8.